The Netherlands' Nuclear Research and Consultancy Group (NRG) has completed a major milestone irradiation test of molten nuclear fuel salts in its High Flux Reactor at Petten 37 mi (60 km) north of Amsterdam. The first test of its kind since the ones carried out at Oak Ridge, Tennessee in the 1960s, its purpose is to learn more about the safe operation of a future Molten Salt Reactor (MSR).

First developed in the United States in the 1950s and '60s, MSRs differ from conventional light-water nuclear reactors in a number of significant ways that make them potentially a safer and more efficient alternative. This is because, though a light-water reactor and an MSR work on the same principle of nuclear fission, they have a fundamentally different engineering design.

In a light water reactor, nuclear fuel is enriched uranium or plutonium set in zirconium-alloy-clad rods immersed in water. This water acts as both the moderator and the coolant for the reactor. When a neutron strikes an atom of uranium or plutonium, it splits, releasing energy and additional neutrons that strike more atoms in a chain reaction, like throwing a ping pong ball into a room filled with mousetraps with ping pong balls perched on each one. The water moderates the reaction by slowing down the neutrons, increasing the chances that they will strike an atom.

It's a system that has been in use for over 60 years and, despite its public reputation, has the best per kilowatt hour safety record of any major power source. However, light-water reactors do have a number of disadvantages due to the fact that they rely on the water being kept at very high pressures and temperatures. But by replacing water with salt, a number of advantages appear.

There are several different molten salt reactors but in one of the most common being developed, the layout is essentially reversed. Instead of water surrounding fuel rods, the fuel is mixed in with salt, which has been heated to the point where it melts and flows like a liquid – somewhere in the neighborhood of hundreds or even thousands of degrees Celsius. Instead of fuel rods, there are rods of graphite, which act as a moderator and control the strength of the reaction.

Though never developed into a practical commercial power plant, research into molten salt reactors at various centers around the world have demonstrated a number of advantages. For example, they can be fueled by a wide variety of elements, including relatively abundant fissionable ones, like thorium, and the waste products are much more active, so they are reduced to safe radioactive levels faster than the spent fuel from conventional plants that haven't been reprocessed.

In addition, the molten salt reactor doesn't need to be shut down for refueling. Instead, the old fuel can be filtered out by a chemical plant and fresh fuel pumped in. They also operate at lower pressures and they also don't produce steam or potentially explosive hydrogen gas – both of which are major safety issues for conventional reactors – so there's no need for heavy pressure vessels. And because the MSRs works at a higher temperature than conventional reactors, they are more efficient and smaller in size.

Another safety factor is that molten salt expands under heat, so if there is a runaway reaction, the expansion shuts it down. In a serious emergency, MSR designs include drainage tanks into which the salt automatically pours under the force of gravity, separating it into smaller units and killing the reaction dead.

However, there are a number of disadvantages to MSRs. the salt is not only hot it's also highly corrosive and it has to be in direct contract with pumping equipment. This risks both corrosion and radioactive damage, like embrittlement.

Because of this, NRG is conducting a series of irradiation tests to see how a nuclear environment affects molten salt nuclear fuels. According to the company, this began with its SALIENT-01 experiment in 2015 as part of a thorium reactor concept. Current work includes research on construction materials and the processing and purification of molten salt and residual products.

Further irradiation tests are scheduled, including the study of what happens with nuclear fuel salts cool down to near room temperature in a radioactive environment. This will be followed next year by corrosion tests of candidate reactor alloys in the High Flux Reactor.

"Completing our work inside the reactor means we can now examine the irradiated salt more closely in the NRG labs," says Ralph Hania of NRG. "This means we’ll really be able to see how the salt responds to irradiation in the reactor."

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David Szondy is a freelance journalist, playwright, and general scribbler based in Seattle, Washington. A retired field archaeologist and university lecturer, he has a background in the history of science, technology, and medicine with a particular emphasis on aerospace, military, and cybernetic subjects. In addition, he is the author of a number of websites, four award-winning plays, a novel that has thankfully vanished from history, reviews, scholarly works ranging from industrial archaeology to law, and has worked as a feature writer for several international magazines. He has been a New Atlas contributor since 2011.

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piperTomSeptember 21, 2019 07:59 AM

The article lists thorium as fissionable. That's a reasonable simplification in this context, but the abundant thorium isotopes used in reactors are regarded as fertile, and not directly fissionable. An isotope is fertile when it is readily converted to a fissionable isotope by irradiation. The extra step is important because it greatly reduces the chance of an uncontrolled chain reaction.

UsernameSeptember 21, 2019 10:32 AM

In 1968, Nobel laureate and discoverer of plutonium, Glenn Seaborg, publicly announced to the Atomic Energy Commission, of which he was chairman, that the thorium-based reactor had been successfully developed and tested.
The reason they were not implemented is that they could not be used as part of the nuclear weapons manufacturing line.

Robert BernalSeptember 22, 2019 09:39 AM

Awesome! Perhaps all the areas that don't have good solar or wind won't have to be dependant on others for their non fossil power. However, I was hoping that the world could overcome this silly nationalism and connect with long powerlines. This much better nuclear would help to rid poverty. And it should be cheaper than wind, solar and batteries (you would think). How much material and energy input needed to build a, say 100 Mw reactor, compared to that much of solar, wind and batteries? Also, no huge pressure vessel, just a hardened slab.
I don't understand why enviros are against nuclear. They are also against large wind and solar, too!
Here's a list of links that show that environmental leaders really do not take serious the need to transition from fossil fuels. Do they have an inferior motive?
Anti clean energy enviro's...
https://www.sandiegoreader.com/news/2015/mar/19/ticker-boulevard-environmentalists-against-solar/#
https://www.mercurynews.com/2017/07/21/giant-solar-project-reduced-due-to-environmentalists-opposition/
https://www.citylab.com/environment/2015/07/the-environmentalist-case-against-100-renewable-energy-plans/398906/
http://science.time.com/2012/02/02/exclusive-how-the-sierra-club-took-millions-from-the-natural-gas-industry-and-why-they-stopped/
https://www.theregister.co.uk/2008/10/22/fusion_greenpeace_no/
https://www.latimes.com/environment/story/2019-08-27/los-angeles-solar-energy-storage-cheap
And on advanced nuclear (read the last few paragraphs) https://www.nytimes.com/1989/09/25/business/experts-see-nuclear-energy-as-cure-for-global-warming.html
https://www.scientificamerican.com/article/major-u-s-offshore-wind-projects-still-face-hurdles/

Paul Muad'DibSeptember 22, 2019 12:27 PM

Water cooled reactors came from submarines, the molten salt reactor came from an airplane.

Mzungu_MkubwaSeptember 23, 2019 06:20 AM

Didn't realize that hydrogen is a byproduct in current reactors. Makes sense if water's role is (partly) "slowing down the neutrons" as I'm sure this can result in some splitting of the oxygen and hydrogen, right? I wonder if they employ fuel cells to convert this into electricity "on the side"? As to the corrosive nature of the molten salts, could they just pipe it and pump it through concrete and/or ceramics? Surely these materials are less susceptible?

AbhijithSeptember 23, 2019 10:45 AM

The way you have explained nuclear fission and the function of moderator is interesting. However, certain points are to be mentioned. First of all Thorium is not fissile. Its isotope is a fertile one which has to be converted into a fissile isotope inside a breeder reactor in order to use it as a fuel. Secondly, there are PHW reactors like the CANDU reactor in which fuel rods can be changed without complete shutdown. Also there is no mention about the handling of waste from MSR.

GannetSeptember 24, 2019 01:30 AM

"this began with its SALIENT-01 experiment in 2015" and then "This will be followed next year by corrosion tests of candidate reactor alloys..."
So, not ready to power the world in any hurry. And next year renewables will be yet cheaper - I wouldn't want to have any investments waiting on this.

BeinThayerSeptember 24, 2019 10:17 AM

"... They also operate at lower pressures and they also don't produce steam ..."
It would be interesting to know how it is proposed for this reactor to efficiently produce electricity without producing steam.

guzmanchinkySeptember 27, 2019 05:08 PM

I so hope this works out. I've been wondering why we haven't used LFTR for so long.

CraigPOctober 3, 2019 11:26 PM

Mzungu_Mkubwa: Regarding Hydrogen, my understanding is that light-water reactors cause H2O molecules to split into oxygen and hydrogen - therefore pressure-water reactors utilise what is called a "recombiner" (not sure of the spelling) which works to literally recombine the oxygen and hydrogen back into water to avoid any buildup of either gas - when the tsunami came and knocked out the reactor in 2011, the recombiner's worked for 8 hours until the batteries powering them ran out - then the hydrogen gas was left to build up and the explosion that occurred at part of the reactor building (not sure exactly where, but which was caught on film) was a hydrogen explosion.
BeinThayer: The reactors do not produce steam because there is no water inside them - only the molten salt. However they would work by circulating the hot molten salt through a heat exchanger - in some proposals, to exchange with non-fuel molten salt (like the stuff they use a solar power plants to hold high temperatures to use for a few hours after sundown) and that non-fuel molten salt would then go through another heat exchanger where it transfers the heat to water to create steam - so the steam is created by the heat that has been generated but that is different to the current pressure water reactors which have the high-temp/high-pressure water inside them ( i.e super-heated steam "yearning to be free") in the reactor WITH the solid fuel - my understanding is the Chernobyl explosion back in 1986 was literally an extremely powerful "steam" explosion that as collateral damage couldn't help but blast bits of the pulverised fuel rods out into the atmosphere.